1. Field of the Invention
This invention relates to gas discharge surge arresters and more particularly to an arrester of the above type in which the effect on the breakdown voltage of external metal grounded structures is essentially minimized.
2. Description of the Prior Art
Surge voltage arresters of the cold cathode type serve as the primary arrester and source of protection in various line protectors. Such arresters typically include some type of air gap as a back-up or ventsafe feature in the event of the failure of the gas tube arrester.
One example of such a surge voltage arrester is that described in U.S. Pat. No. 4,241,374 entitled "Surge Voltage Arrester With Ventsafe Feature" which issued on Dec. 23, 1980 in the name of Alexander G. Gilberts and is assigned to the assignee as is the present invention. As described therein the gas tube arrester is housed within a metallic cup. The sidewall of the cup has a diametrically enlarged annular cylindrical end portion near its open end. This enlarged end portion is used to define a secondary air gap, i.e., the enlarged portion defines the ventsafe portion.
As is well known the gas tube has a predetermined breakdown voltage which is dependent on a number of factors including the spacing of the electrodes in the tube. When, prior to inseration in the cup, the breakdown voltage of the tube is tested it has been found that breakdown occurs at essentially the same voltage independent of which electrode is made the cathode. When the breakdown voltage is tested again after the tube has been inserted in the cup, it has been found that the voltage at which the tube breaks down now depends on which electrode is made the cathode. In other words, when the tube is inserted in the cup the breakdown voltage in both directions is not the same and in fact there is a substantial differential between those breakdown voltages.
In most applications where such ventsafe surge arresters are used, this differential in breakdown voltage is of little consequence. There are, however, some applications for such surge arresters where the specifications require that the arrester have a narrow range for its breakdown voltage. Therefore, it is desirable to minimize the asymmetrical effect that the cup has on the arrester's breakdown voltage.